Introduction to Photography Exposure Terms – Part 4: Exposure

Written by Marc B

Last week we introduced beginners to ISO, and in previous week’s we introduced aperture and shutter speed. This week is our fourth and final part in our Introduction to Photography Exposure Terms blogs; we’re introducing exposure ie. how to bring it all together. 

The correct exposure for an image is a combination of shutter speed, aperture value, ISO setting and scene brightness, with each of these values having an effect on the exposure. 

If any of the values of these settings change, it is necessary to change one or more of the other values to make up for it. 

For example, where an exposure of 1/125^th at f8 and 200 ISO is the correct exposure for the scene brightness, and you choose to change the aperture to f5.6, more light will pass through to the sensor so the image will now be over exposed.  To accommodate this change you will need to adjust another setting. 

Assuming you couldn’t change the light levels in the scene, options would be:

·           A faster shutter speed (in this case 1/250^th).  This would halve the amount of light that the shutter would let in and therefore bring the exposure back to the original value, making up for the fact that the aperture is letting in double the light as it was originally.

·           Lower the ISO setting to 100 ISO.  This would lower the sensitivity of the sensor by half and would also make up for the original opening of the aperture being opened up.

·           A combination of both.

Below are some examples of photos taken at the correct exposure, then under exposed by two stops and over exposed by two stops.

This is the correct exposure according to the camera - a good overall exposure with some loss of detail in the brighter and darker areas.

This photo has been under exposed by two stop of light, resulting in a much darker image as compared to the image above. There is significant loss of image information in the darker and mid-toned areas.

The below photo has been over exposed by two stops, resulting in a much brighter image as compared to the first image. There is significant loss of detail in the light areas.

 

 

This diagram shows the relationship between all the values that need to be considered to get the exposure right on a camera.

The most important thing of course, is to experiment.  Get out of Auto mode; see what happens when you change the camera settings.  It costs nothing and you have everything to gain.  If you have any questions about any of the information you’ve learnt over the course of this Introduction to Photography Exposure Terms blog posts, feel free to contact us or ask in the comments section below.

We are always happy to talk cameras!

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Introduction to Photography Exposure Terms – Part 3: ISO

By Marc B

Last week we introduced beginners to aperture, and the week before we wrote about shutter speed. This week (in Part 3 of the Introduction to Photography Exposure Terms), we’re introducing ISO or sensitivity.  Using these settings effectively, will help you achieve the right exposure and therefore the right look and feel for the photo you want to take. Remember to play around with these settings and experiment. You will learn with practice, so start shooting!

ISO or Sensitivity

The ISO setting on a DSLR is used to increase or decrease the sensor’s sensitivity to light.  Current ISO sensitivity ratings are based on the original sensitivities of film i.e 100, 200, 400, 800 etc.  The ability to change the ISO quickly is very useful, as it allows you to adjust your photo according to the available light in the area you are shooting. 

If the shutter speed is too slow and you have no alternative then you can increase the ISO which will allow you to have a faster shutter speed with the same amount of light.  Be aware that as you increase your ISO you also increase the appearance of ‘digital noise’ in your image. ‘Digital noise’ shows up as a messy multi-coloured haze across the image and is particularly noticeable in block areas of colour or shadow.  The ‘digital noise’ gets worse as you increase the ISO.

Typically the ISO has a range from 100 ISO up to 25600 (or more, depending on your camera). When talking about ‘general photography’, most photos are shot in the range of 100 to 1600 ISO.  The best ISO to choose depends on a number of variables. One of these variables is when the content of the image is the absolute first priority (eg a time critical photo for a news story where the technical result is not as important as ‘just getting the shot’). In this case, a high ISO is fine. It will ensure the photo has a better chance at being sharp.

 

Examples

The photo on the left shows a cropped area of an image shot with an ISO of 100.  It is very clean with good contrast and next to no digital noise.

The photo on the right was shot at almost exactly the same time but with the ISO set to 6400.  The difference is less contrast and a lot of ‘digital noise’.

 

The ISO setting can be a powerful tool when deciding on your exposure settings.  Adjusting the setting will allow you to move to a faster or slower shutter speed while keeping your aperture consistent. Keep in mind that as you increase the ISO you also increase the appearance of digital noise in your image with a corresponding decrease in image quality.

Experiment and have fun!

Next week, Part 4 of our ‘Introduction to Photography Exposure Terms’ will be on Exposure (bringing it all together).

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